Definition
An electrical position-indicating system in which a transmitter unit, mechanically linked to a moving part of the aircraft, is electrically connected to one or more indicator units in the cockpit so that the indicator's pointer accurately mirrors the angular position of the transmitter shaft. Selsyn systems are used for remote indication of items such as flap position, landing gear position, and trim tab position.
Plain English
A simple electrical setup that lets a needle on a cockpit gauge follow the exact position of something moving elsewhere on the aircraft, like the flaps or trim tab, even though they are not mechanically connected.
Context Anchor
Seen in aircraft maintenance when checking remote position indicators, such as indicators for flaps, trim, valves, or other parts that move away from the cockpit.
Derivation
Selsyn is a contraction of 'self-synchronous,' meaning the transmitter and indicator stay in step with each other automatically through their electrical connection. Knowing this helps you remember the system's job: keeping a remote indicator synchronized with a moving part.
Why Pilots Care
Provides accurate real-time cockpit readings of control surface positions and other variables that would otherwise require direct mechanical connections.
Analogy
It is like turning one knob and having a second knob across the room turn to the same position, even though the two are connected by wires instead of a rod.
Intuition Check
Do not think of a Selsyn system as the moving aircraft part itself. It is the electrical system that reports that part’s position at another location.
Example Sentence 1
The flap position indicator is driven by a selsyn system, so the cockpit pointer moves in step with the flaps as they extend.
Example Sentence 2
A failed Selsyn transmitter can produce incorrect readings on the cockpit gauge even when the actual surface is in the correct position.